Electron paramagnetic resonance evidence for the occurrence of hydrogen and/or proton transfer between C7, C8 and C11 n-alkanes and their cations in irradiated CCl3F matrices at 77 K

Abstract

After γ-irradiation of heptane and octane at low concentration in CCl3F at 77 K, only the EPR spectrum of the corresponding radical cations is observed. At higher concentrations, the spectrum of alkyl radicals also appears. Clear alkyl radical features are also observed after irradiation of 1 mol% undecane in CCl3F. In contrast, alkane radical cations but no alkyl radicals are observed after irradiation of heptane, octane and undecane in CCl3CF3 and other saturated chlorofluorocarbon matrices at concentrations where alkyl radicals are already very prominent in CCl3F. Different mechanisms for the formation of these alkyl radicals are considered, viz. direct radiolysis of the alkane solutes, formation as a result of electronic excitation energy transfer and formation via the corresponding alkane radical cations that are formed by hole transfer from matrix cations. In the latter case, unimolecular deprotonation, charge neutralization by electron tunnelling and hydrogen/proton transfer with alkane molecules are taken into consideration. It is concluded that the hydrogen/proton transfer reaction (RH˙++ RH → R˙+ RH+2) mainly accounts for the observed alkyl radicals. The results further indicate that in CCl3F at 77 K alkanes are present as small aggregates, to which hole transfer from matrix cations still occurs efficiently.